It is our intent to study the biological effects of gamma-irradiation on the snail hosts and miracidia of Schistosoma mansoni. Utilizing normal and irradiated Biomphalaria glabrata and normal and irradiated Schistosoma mansoni miracidia, it is proposed: 1) To determine the effects of radiation on the susceptibility of the snail to become infected with S. mansoni. 2) To utilize the irradiated-snail model as a means of investigating cellular defense mechanisms in host snails. 3) To determine if non-susceptibility caused by irradiation may be passed to subsequent generations of snails, and thus create a genetically nonsusceptible strain. 4) To determine if attenuated cercariae can be produced by infecting snails with irradiated miracidia or by altering the host snail's physiology through radiation, and if such cercariae can be used to induce immunity in experimental animals.